Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
noirdesir Forum Pro • Posts: 13,586
Re: They're Relative to Wide Open

Marianne Oelund wrote:

noirdesir wrote:

I presume all your measurements are relative to wide-open. If you see a consistent under- or overexposure, could it be that the f-stop wide open isn't exactly the nominal one and that thus the stopped down values are the correct ones and the aperture wide open is the "inaccurate" one?

In other words, can the aperture error compensate for the efficiency loss when the lens is stopped down? In some cases yes, but unfortunately that is purely academic, and of little help to the user, as the image exposure will still be thrown off. If a manufacturer were to intentionally do this to "correct" for light loss, they would be doing a disservice to their customers!

Just to make sure we are talking about the same thing, I meant that if a f/2 lens actually was an f/2.1 lens and you tried to measure the accuracy of f/2.8 stop you would measure a delta of 0.8 stops between them or in other words conclude that the f/2.8 aperture is 1/5 stop too bright (always assuming naturally that the ratio between f and t-stop is the same for both apertures which is what you assumed already). Now, if the camera new that a lens actually had a f/2.1 aperture wide open it would adjust a nominal f/2 and 1/100 s not to f/2.8 and 1/50 s but to 1/56 s (eg, in aperture-priority mode).

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